DESCRIPTION: The photoreceptor cells of the retina carry out the essential task of translating light into electrical signals, which are then transmitted to retinal bipolar neurons for further analysis before being communicated to the rest of the visual system. All visual information must pass through this obligatory pathway, which involves two synaptic relays, one from photoreceptors to bipolar cells, and another from bipolar cells to third-order neurons of the inner retina. Both of these through-conducting synapses are specialized for continuous release of chemical neurotransmitter, which is required for the proper transmission of visual information. Despite their crucial role in vision, these unique photoreceptor and bipolar-cell synapses remain poorly understood, and it is not yet known how they are able to support high rates of neurotransmitter release for prolonged periods, while other synapses in the brain fatigue rapidly during sustained stimulation. This project will use a combination of cellular electrophysiology and high-resolution fluorescence imaging to provide the first complete picture of the synaptic vesicle cycle at the special synapses of retinal bipolar neurons and photoreceptor cells. The vesicle cycle can be broken down into several components: rapid fusion of synaptic vesicles to initiate transmitter release, resupply of releasable vesicles to support continued release, movement of vesicles to and from reserve and releasable pools, compensatory endocytosis to retrieve fused vesicles, and regeneration of new releasable vesicles after endocytosis. The specific aims of the project exploit the ability to tag synaptic vesicles and synaptic active zones with distinct fluorescent labels to observe directly the vesicle movements that underlie all these components of the vesicle cycle. Then, genetic manipulations and selective protein interference will be used to unravel the molecular machinery that gives retinal synapses the unique ability to release neurotransmitter continuously and allows them to perform their central role in transmitting visual signals. The results will provide essential new information about how the visual signals arising in the photoreceptor cells are ultimately transmitted to the rest of the brain.